The quasi-resonant operation is one of the soft-switching techniques. In the switching power converter, a higher reflected voltage will help to achieve the quasi-resonant operation. Therefore, the switching power converter can be operated in higher switching frequency with a high efficiency for reducing the size of the power transformer. The related prior art can be found in U.S. Pat. No. 7,426,120, entitled “Switching control circuit having a valley voltage detector to achieve soft switching for a resonant power converter”. FIG. 1 is a conventional dual switches Flyback power converter. A power transformer 10 includes a primary winding NP and a secondary winding NS. The primary winding NP of the power transformer 10 has a first terminal and a second terminal. The secondary winding NS of the power transformer 10 is coupled to generate an output voltage VO of the power converter through a rectifier 40 and an output capacitor 45. A first terminal of the secondary winding NS is coupled to an anode terminal of the rectifier 40. The output capacitor 45 is coupled between a cathode terminal of the rectifier 40 and a second terminal of the secondary winding NS.
A high-side transistor 20 is connected from an input voltage VIN to the first terminal of the primary winding NP. A positive terminal of an electrolytic capacitor 50 is connected to the input voltage VIN and a negative terminal of the electrolytic capacitor 50 is connected to a ground. A low-side transistor 30 is connected from the second terminal of the primary winding NP to the ground. A first diode 35 is connected from the second terminal of the primary winding NP to the input voltage VIN. A second diode 25 is coupled from the ground to the first terminal of the primary winding NP.
A low-side switching signal S1 is coupled to drive the low-side transistor 30 and a high-side switching signal S2 is coupled to drive the high-side transistor 20. When the transistors 20 and 30 are switched off simultaneously, the energy stored in the leakage inductor (not shown in the figure) of the power transformer 10 (the primary winding NP) will be recycled to the electrolytic capacitor 50 through the diodes 25 and 35 for power saving. Furthermore, a reflected voltage of the power transformer 10 will be generated across the primary winding NP when the transistors 20 and 30 are switched off. The reflected voltage is correlated to the output voltage VO and the turn ratio NP/NS of the power transformer 10. If the reflected voltage is higher than the input voltage VIN, the energy stored in the power transformer 10 will be discharged to the electrolytic capacitor 50 instead of delivering to the output voltage VO when the transistors 20 and 30 are turned off. The drawback for this power converter shows that the output voltage VO will decrease when the input voltage VIN is lower than the reflected voltage.